Only if you believe it works.
Yes, the GM is free to change the rules on the player without warning. Or, if you want to stay within the rules, arbitrarily have a powerful demon sneak into the magus's lab and mess with their lab so the spell only 'looks' like it works.
Pretty close, though Chthonic Magic doesn't register as Infernal to magical or faerie investigation most of the time, so they can escape being accused of infernalism - much like magi with the Summoning power.
Tha rulez on RoP:TI p.122 specify, that Cleanse the Verminous Vis "does not operate quite how the Venatores think that it does".
So a character using this spell is not supposed to know how it really works - and if her player games the system, he should not blame troupe or SG for the outcome.
Cheers
Yeah, it doesn't operate in that it takes up to three applications to fully cleanse the infernal vis. All it takes is for a single player being cautious to test the cleansed vis prava with a minor spell to discover that it became vis sordida (that +5 instead of +2 is rather noticeable), or even looking for a character with decent ranks of Sense Unholiness to take a look, and then triple applications will become the norm.
And players who think, that their characters know this, might then find out that repeated application of the spell does not do zilch any more. 
Seriously: this you must sort out with your troupe, not on the forum.
Cheers
Changing the rules on the players is a Bad IDEA[sup]TM[/sup]. Adding a wrinkle, or an exception for them to discover may be fine, but it can cause acrimony within the troupe.
Consider the flame wars that have developed here about the Aegis needing to penetrate. Both sides have compelling reasons for believing the Aegis works "their" way, but their understanding of the underlying rules is vastly different. When you have players expecting one thing and getting another, and it happens in play it's even worse, it's just like changing the rules.
The GM is not free to change the rules without warning. A simple rules change can invalidate entire character concepts or unintentionally nerf mechanical builds into uselessness; this isn't something that should be done without warning or consultation.
I can't tell if you're trying to correct me or the sentiment I was attempting to mock with sarcasm.
Please use sarcasm tags then.
As I was responding to Richard Love and One Shot who literally were suggesting that Cleanse the Verminious Vis might, without reason, not be a valid and safe option for infernal vis; I would presume you'd try to correct them and not me...
Which ignores the point that the characters should not know exactly how the spell works. Nor should they know how/whether vis is tainted.
So, one isn't changing the rules, so much as one is adding uncertainty to a situation that their characters would not rightly know. Players may well know that more than one casting is necessary, but there's little to suggest that characters know that, since even the Venetores think one casting is sufficient, based on the description of the spell...
And as I said before, it's not even out of character for a player to be cautious and find out that vis prava becomes sordida; as +5 per pawn is a notable bonus, and passing it by someone with Sense Unholiness is likely. So, unless you outright change the rules, it's likely for them to find out that it takes multiple applications of the spell.
Which ignores or presumes the character would divulge that information I the first place. Such a character who willingly uses such Vis and can tell that it adds more power (a presumption that the rules don't comment on) isn't one who could be trusted. That could be the point, though. Given that the Venetores Think the Vis is pure after one spell casting is telling. It is almost as if someone is lying to them...
One might assume that vis that is tainted by the infernal will ALWAYS ping on Sense Unholiness even if you use Cleanse the Verminous Vis upon it 100 times. IOW just because it's no longer dangerous for hermetic use, vis that has been harvested from corrupted beasts, demons and infernal auras will always be unholy.
So you need to hide information like whether magi can even notice the increased bonus from a pawn of sordida, just to make sure they don't discover that the Cleanse spell needs multiple applications. And for Sense Holiness & Unholiness to not be helpful in noticing the reduced presence of the infernal in cleansed vis (prava>sordida>infesta>'clean'), it needs to have virtually no resolution, possibly retroactively if you have ever let characters notice an infernal aura being stronger at night or something to that effect.
Seems like quite a bit of effort to either not let players figure out the world around them or change the rules behind their back.
I don't have a problem with magi figuring all this out in-play. And obviously the players know if they read the relevant bits of RoP:I.
It means:
- The magus needs to have vis that they think is infernal.
- The magus needs to have the Cleanse spell (and think that it is morally right and proper to use cleansed vis).
- The magus needs to have a way of detecting that the Cleanse spell hasn't actually worked (other than by simply using the vis).
- The magus needs to decide that the solution is not that the Cleanse spell doesn't work, but that he needs to cast it again.
- He then needs to conclude that he actually needs to cast the spell a third time...and figure out that it worked this time
This does seem to require quite a lot of moving parts and suspicion and effort and a mix of skills that implies several magi working together on something inherently near the border of code infringing (which admittedly sounds exactly like a PC covenant).
Any individual magus can be anywhere on the stepped continuum of "infernal vis cannot redeemed"->"infernal vis can be cleansed"->"the cleansing spell doesn't work"->"infernal vis can be cleansed if you do it right", and the existence of the next step on the continuum doesn't seem something that would be apparent in-character until it was reached. RAW seems to imply that most magi are on step 1, or at best step 2. PC magi can be anywhere on the continuum, of course.
Reading the Ability writeup for Sense Holiness and Unholiness, there doesn't seem to be any mention of resolution. It senses 'good' and 'evil', and the user can be overwhelmed by powerful auras. So yes, infernally sourced vis might always ping as 'evil'. Heck, some regular vis might ping as 'evil' too, depending on the circumstances, as evil does not necessarily mean Infernal and good does not necessarily mean Divine, YMMV.
Note casting Cleanse the Verminous Vis is dangerous as all heck to begin with, as it incurs all the problems of casting with Infernal vis. Which means if you're casting on Vis Prava, you roll for a botch automatically with +2 botch dice per pawn. Small wonder magi would not want to cast this repeatedly on vis they find. Vis Prava means you get to auto-roll for botch, then Vis Sordida means you get +3 botch dice per pawn if you roll a zero on the spell, and Vis Infesta gets you extra botch dice as well. That's a lot of risk taken to clean up some vis.